1. Field of the Invention
The present invention relates to an image processing method and an image processing apparatus for forming a high quality gray scale image.
2. Description of the Related Art
As an example of a recording apparatus that is capable of outputting a color image, there is an ink-jet recording apparatus having a plurality of color inks. When an image is formed by using subtractive color mixture as is the case with the ink-jet recording apparatus, three basic colors consisting of cyan (C), magenta (M), and yellow (Y) are generally used. Use of this combination of colors allows representation of not only hues of cyan, magenta and yellow but also various other hues. For example, red (R) can be represented by mixing magenta and yellow. By gradually changing the ratio of amounts of inks mixed together, colors over an entire color space can be substantially represented.
However, in practice, it is very difficult to find basic color materials (C, M, and Y) having colors located at ideal coordinates in the color space. Coordinates in the color space of respective color materials deviate more or less from their ideal coordinates, and the deviation varies depending on the type of a recording medium used. Further, when more ink is employed to increase the density of a color, the resultant locus in the color space does not necessarily linearly extend toward high color saturation, and the saturation tends to decrease with increasing amounts of ink in a range higher than a particular value of the density of the color. The deviation of the color of a recorded image from ideal coordinates in the color space can occur in many types of recording apparatuses including the ink-jet recording apparatus. Hereinafter, such a deviation will be referred to as a “recorded color deviation”.
In a recording apparatus in which recorded color deviations can occur, recording data is corrected depending on the characteristics of inks and/or a recording medium such that a recorded color becomes as close to an ideal color as possible. However, of various hues to be represented, black and gray (achromatic colors) are difficult to adjust. In the gray scale, even a slight increase/decrease in amount of ink can result in a large change in hue perceptible by human eyes. Although black can be represented by mixing three basic colors, it is difficult to obtain black with a sufficiently high density even if maximum possible amounts of the three color inks are used. In recent ink-jet recording apparatuses, to avoid the difficulty described above, black ink is added to the three basic colors. In such an ink-jet recording apparatus, when a gray scale is represented, only the black ink is used or basic color inks are used together with the black ink. Japanese Patent Application Laid-Open No. 2000-198227 discusses the technique in which pigment ink is used for the black color while dye inks are used for the basic three colors in order to ensure the black color density.
FIG. 1 shows output values of respective color inks used to record a gray scale image in an ink-jet recording apparatus discussed in Japanese Patent Application Laid-Open No. 2000-198227. In FIG. 1, the horizontal axis represents values of a luminance signal varying from white (255) to black (0). As the luminance signal value is closer to 255 (white) at the left end, the output result becomes a lower density, while as the luminance signal value is closer to 0 (black) at the right end, the output result becomes a higher density. Accordingly, this horizontal axis represents a range equivalent to the entire density range (entire gradation range) from the lowest density to the highest density of a gray scale image to be actually output. Meanwhile, the vertical axis represents output density signal values (0 to 255) of respective ink colors to be output in response to respective luminance signal values. The output density signal value is any value in the range from 0 (white) representing the lowest density to 255 (black) representing the highest density. As can be seen from FIG. 1, in a low density range, three colors C, M, and Y are used to form a gray image. The output signal values of the three colors are different from one another to make the colors well balanced so as to prevent the “recorded color deviation”. In FIG. 1, in the high density range, use of a black ink (K) is started, and in the highest density range (luminance signal=0), the output density signal value becomes around 128. In FIG. 1, it is assumed that pigment ink is used for the black ink. However, the shapes of the general output curve in the case to record a gray scale image are not limited to the shapes shown in FIG. 1. Use of the black ink may be started at a value greater or smaller than that in FIG. 1. Further, the output density signal value for black at the highest density (luminance signal value=0) may reach, for example, 255. The output density signal values for color inks need not be monotonically increased as shown in FIG. 1. The output density signal values for color inks may be gradually decreased after use of the black ink is started.
In recent ink-jet recording apparatuses, there is a demand for smooth and high calorimetric images that are comparable to silver halide photographs, and to cope with this demand, various techniques have been developed. In comparison with silver halide photographs, what has been most troublesome in the conventional ink-jet recording apparatuses is granularity of an output image that is perceptible to a user. Granularity refers to a visually perceptible rough texture that appears in an image due to ink dots used to record the image on a recording medium. In essence, images with visible granularity are considered low quality compared with silver halide photographs.
To reduce such granularity, many recent ink-jet recording apparatuses use a plurality of inks that are similar in color but different in density.
FIG. 2 shows output density signal values of respective ink colors employed by an ink-jet recording apparatus using a plurality of inks that are similar in color but different in density, in which output density signal values are plotted in a manner similar to that in FIG. 1. Herein, in addition to cyan (C), magenta (M), yellow (Y), and black (K), light cyan (LC) and light magenta (LM) with low color material density are used. As shown in FIG. 2, in the low density range, the gray scale image is formed using three colors of LC, LM and Y. In the process where density increases gradually from low density to high density, if the high-density inks are used, dots are formed sparsely and a visible granular texture appears. Therefore, low-density inks are used to avoid such granularity. When low-density inks are used, ink dots formed on a recording medium are not easily perceptible.
For the purpose to reduce granularity in similar manners, many ink-jet recording apparatuses have been proposed in which inks that are of the same color but different in discharge amount (dots that are of the same color but different in size) are used to form an image (for example, Japanese Patent Application Laid-Open No. 10-16251). For example, to reduce granularity, small cyan (SC) and small magenta (SM) are used instead of light cyan (LC) and light magenta (LM). These small cyan (SC) and small magenta (SM) are used in a manner similar to light cyan (LC) and light magenta (LM) shown in FIG. 2 so as to reduce granularity.
As shown in FIG. 2, in the middle density range, the output signal values of LM (or SM) and LC (or SC) become near their maximum values, and densities still higher than these values cannot be obtained by using any combination of these inks. On the other hand, since an image recorded on a recording medium is fully filled with many dots, granularity due to a single dot is not easily perceptible. Therefore, from this stage, inks of C, M, and further K are gradually added, and density can be increased while maintaining low granularity. At the same time, the output values of LC (or SM), LM (or SC), and Y are gradually decreased. Finally, the output value of K exceeds the output value of any other ink, and black of high density and a preferable hue can be represented.
In recent ink-jet recording apparatuses, there is a demand for image quality that are comparable to that of silver halide photographs, and this is also true of a gray scale image. The results of strenuous examinations by the present inventors to obtain a gray scale image at a level comparable to silver halide photographs have found that there are cases where “recorded color deviation” and “color transition” cannot be avoided by the conventional monochrome mode shown in FIG. 1 and FIG. 2. Namely, in recent years, ink droplets have been made smaller for high image quality, but when ink droplets are made small, variations in ink discharge amounts affect greatly upon image quality. Especially in a gray scale image, image quality deterioration due to variations in discharge amount is conspicuous, and in the conventional monochrome mode shown in FIG. 2, “recorded color deviation” occurs, deteriorating the gray balance to an extent that cannot be ignored. Further, the amount and the direction of recorded color deviation are uncontrollable, and the recorded color deviation can cause a phenomenon where an abrupt transition occurs in monotonic gradation changes and hue changes (hereinafter, referred to as “color transition”). Especially, as described with reference to FIG. 2, such a “color transition” tends to occur in a range in which dominant ink is switched from the low density range to the high density range.
As described above, the conventional monochrome mode as shown in FIG. 2 cannot suppress or reduce “recorded color deviation” and “color transition”. Users typically dislike such “recorded color deviation” and “color transition” in a gray scale image. Therefore, it is indispensable to resolve the “recorded color deviation” and “color transition” problems to achieve a higher quality gray scale image.